The present invention relates to a liquid crystalline compound and a liquid crystal composition, more specifically, to a liquid crystalline compound having a difluoropropyleneoxy group as a bonding group, which shows physical properties particularly suited as a component of a liquid crystal composition for a TN mode, an STN mode, a TFT mode and an OCB mode, a liquid crystal composition comprising the same and a liquid crystal display element containing this liquid crystal composition. A liquid crystalline compound herein means a compound showing a liquid crystal phase and also a compound which does not show a liquid crystal phase but is useful as a component of a liquid crystal composition.
A liquid crystal display element makes use of optical anisotropy and dielectric anisotropy of a liquid crystal substance and is classified into various modes such as a twisted nematic (TN) mode, a dynamic scattering (DS) mode, a guest-host (GH) mode, a xe2x80x9cdeformation of aligned phases (DAP)xe2x80x9d mode, a super twisted nematic (STN) mode, a voltage controlling birefringence (VCB, ECB or TB) mode, a vertical alignment (VA) mode, a multidomain vertical alignment (MVA) mode and an OCB mode according to a display mode thereof. Liquid crystal substances suited to the respective modes have different properties.
All liquid crystal substances, regardless of the modes, are required to have the following properties:
1) stability to external environmental factors such as moisture, air, heat and light;
2) a liquid crystal phase in a wide temperature range around room temperature;
3) a low viscosity;
4) a reduced driving voltage when driving a display element;
5) a suitable dielectric anisotropy (xcex94xcex5); and
6) a suitable refractive anisotropy (xcex5n).
Under the present circumstances, however, any single compound satisfying all the above characteristics is not available, and several to twenty or more kinds of liquid crystalline compounds are mixed to prepare a liquid crystal composition, which is used for a liquid crystal display element.
Accordingly, liquid crystalline compounds used as components of a composition have to show good compatibility with each other. Recently, they have been required to be used under various environments, and therefore, to have a good compatibility particularly at a very low temperature.
In recent years, a liquid crystal display element has been required to show higher display performances in a contrast, a display volume, a response time, and the like. In order to meet the requirement, there has been a demand for a display element of an active matrix mode represented by a TFT (thin film transistor) mode mainly in the fields of televisions and viewfinders.
A display element of an STN mode is produced in a simple process at a low cost while having a large display volume, so that it is generally used in the display fields of portable telephones, personal computers and the like.
A recent development in these fields has mainly been in miniaturization and portability of liquid crystal display elements as seen in TV and note type personal computers. Accordingly, liquid crystalline compounds having a low driving voltage, i.e., those which can reduce a threshold voltage, and liquid crystal compositions having a low threshold voltage which comprise the above compounds have been required as liquid crystal materials used in this case.
As known well, a threshold voltage (Vth) is shown by the following equation (H. J. Deuling, et al., Mol.
Cryst. Liq. Cryst., 27 (1975) 81):
Vth=xcfx80(K/xcex50xcex94xcex5)xc2xd
wherein K is an elastic constant of a liquid crystal material, and xcex50 is a dielectric constant in vacuo.
As seen from the above equation, increasing xcex94xcex5 or decreasing K can be considered as a method for reducing Vth. However, it is still difficult to actually control an elastic constant K of a liquid crystal material by conventional techniques, and a liquid crystal material having large xcex94xcex5 has generally been used to meet the requirement. Under such circumstances, liquid crystalline compounds having large As have actively been developed.
As a well-known method for increasing xcex94xcex5 in a liquid crystalline compound, a substituent having a large hi dipole moment such as a cyano group and a trifluoromethyl group may be introduced as a terminal group of the molecule. Also effective is a method of substituting a 1,4-phenylene group constituting the compound with fluorine so that the dipole moment turns toward the same molecular axis direction as a dipole moment in a terminal group. In general, however, the number of fluorine substituted on a 1,4-phenylene group is relative to the viscosity, and a clearing point of the compound is reduced as the number of substituted fluorine increases. Accordingly, it has so far been considered difficult to elevate only xcex94xcex5 while preventing both rise in the viscosity and reduction in the clearing point.
In recent years, liquid crystal display elements have become widely used in information terminals and portable games. These display elements are driven by batteries, and therefore, it is requested that the threshold voltage is low and the power consumption is low from a viewpoint of use for long time. Particularly in order to reduce a power consumption of an element itself, a reflective display element not requiring backlight has actively been developed recently, and increase in use thereof for portable telephones is anticipated. Liquid crystal compositions used for these reflective display elements are required to have a small refractive anisotropy (xcex94n) as well as a low threshold voltage. Accordingly, it is important in this field to develop a liquid crystalline compound having a-large dielectric anisotropy and a small refractive anisotropy as a liquid crystal material constituting the composition. The following compounds (13) and (14) (JP-A 2-233626) can be shown as a representative liquid crystal material for driving a display element at a low voltage, which is used for a liquid crystal display element of a TFT mode: 
wherein R represents an alkyl group.
Both the compounds (13) and (14) have a 3,4,5-trifluorophenyl group at a terminal of a molecule and are expected as a liquid crystal material for driving a display element at a low voltage. However, the compound (13) has a small dielectric anisotropy (xcex94xcex5=about 10) for use in the reflective display element described above, and the compound (14) has a satisfactory dielectric anisotropy (xcex94xcex5=about 12) but has a large refractive anisotropy of about 0.12, so that it is considered difficult to prepare a liquid crystal composition which can sufficiently satisfy the above requirements by using these compounds.
In recent years, novel modes such as an in-plain switching (IPS) mode, a vertical alignment (VA) mode, a multidomain vertical alignment (MVA) mode and an OCB mode have been developed as a mode for overcoming a narrow view angle which is the largest problem of a liquid crystal display element. Among these modes, the VA mode and the MVA mode are particularly excellent in response and have a wide view angle, and further, a high contrast, so that they have been actively developed by respective display makers. Liquid crystal compositions used for the liquid crystal display elements of these modes are characterized in that they have a relatively small refractive anisotropy and a negative dielectric anisotropy. For example, the following compound (15) is reported as a compound showing a large negative dielectric anisotropy (V. Reiffenrath et al., Liq. Cryst., 5 (1), 159 (1989)). 
It can be found from the above literature that the compound (15) shows a large negative dielectric anisotropy (xcex94xcex5=xe2x88x924.1), but it has a large refractive anisotropy (xcex94n=0.18). Accordingly, it is anticipated that the above compound hardly satisfies the requirements of the VA mode or the MVA mode described above.
As explained above, a liquid crystalline compound having a large positive or negative dielectric anisotropy and showing a relatively small refractive anisotropy has been desired.
An object of the present invention is to overcome the disadvantages of conventional techniques described above and provide a liquid crystalline compound which has a large absolute value (|xcex94xcex5|) of a dielectric anisotropy and shows a relatively small refractive anisotropy, a liquid crystal composition comprising this compound which makes it possible to drive display elements of various modes at a low voltage, and a liquid crystal display element containing this liquid crystal composition.
The present inventors have made intensive investigations in order to achieve the above object and, as a result, found that the compound having a difluoropropyleneoxy group as a bonding group represented by Formula (1) has a large absolute value (|xcex94xcex5|) of a dielectric anisotropy and shows a relatively small refractive anisotropy. Further, they have found that a liquid crystal composition comprising this compound is the most suitable material for driving various liquid crystal display elements at a low voltage, and thus completed the present invention.
The present invention comprises the followings.
The first aspect of the present invention relates to:
[1] A liquid crystalline compound represented by Formula (1): 
wherein R1 and R2 each independently represent hydrogen, halogen, a cyano group or an alkyl group having 1 to 20 carbon atoms, in which at least one xe2x80x94CH2xe2x80x94 may be substituted with xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94 but xe2x80x94Oxe2x80x94 is not adjacent to another xe2x80x94Oxe2x80x94, and in which at least one hydrogen may be substituted with halogen; rings A1 to A5 each independently represent a 1,4-cyclohexylene group in which at least one xe2x80x94CH2xe2x80x94 not adjacent to each other may be substituted with xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94, a 1,4-cyclohexenylene group, or a 1,4-phenylene group in which at least one xe2x95x90CHxe2x80x94 may be substituted with xe2x95x90Nxe2x80x94 and hydrogen on the ring may be substituted with halogen; Z1 to Z4 each independently represent a single bond, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94CF2Oxe2x80x94 or xe2x80x94OCF2xe2x80x94; Y1, Y2, Y3 and Y4 each independently represent hydrogen or fluorine; and k, l, m and n each independently represent 0 or 1.
The first embodiment of the present invention is described in the following items [2] to [13].
[2] The liquid crystalline compound represented by Formula (1-1) to (1-6): 
wherein R1, R2, rings A1 to A5, Z1 to Z4 and Y1 to Y4 have the same meanings as described above.
[3] The liquid crystalline compound of Formula (1), wherein ring A3 is a 1,4-cyclohexylene group.
[4] The liquid crystalline compound of Formula (1), wherein both yl and Y3 are fluorine atoms, and both Y2 and Y4 are hydrogens.
[5] The liquid crystalline compound of Formula (1), wherein both y1 and Y2 are hydrogens.
[6] The liquid crystalline compound of Formula (1-1), wherein ring A3 is a 1,4-cyclohexylene group, both Y1 and Y3 are fluorines, and both Y2 and Y4 are hydrogens.
[7] The liquid crystalline compound of Formula (1-1), wherein ring A3 is a 1,4-cyclohexylene group, and both Y1 and Y2 are hydrogens.
[8] The liquid crystalline compound of Formula (1-2), wherein both ring A2 and ring A3 are 1,4-cyclohexylene groups, both Y1 and Y3 are fluorines, and both Y2 and Y4 are hydrogens.
[9] The liquid crystalline compound of Formula (1-2), wherein both ring A2 and ring A3 are 1,4-cyclohexylene groups, both Y1 and Y3 are hydrogens, both Y2 and Y4 are fluorines, and R2 is an alkoxy group.
[10] The liquid crystalline compound of Formula (1-2), wherein both ring A2 and ring A3 are 1,4-cyclohexylene groups; and both Y1 and Y2 are hydrogens.
[11] The liquid crystalline compound of Formula (1-2), wherein both ring A2 and ring A3 are 1,4-cyclohexylene groups, both Y1 and Y2 are hydrogens, both Y3 and Y4 are fluorines, and R2 is fluorine.
[12] The liquid crystalline compound of Formula (1-2), wherein both ring A2 and ring A3 are 1,4-phenylene groups in which hydrogen on the ring may be substituted with fluorine, Z2 is a single bond, both Y1 and Y2 are hydrogens, both Y3 and Y4 are fluorines, and R2 is fluorine.
[13] The liquid crystalline compound of Formula (1-3), wherein ring A3 is a 2,3-difluoro-1,4-phenylene group.
The second aspect of the present invention relates to:
[14] A liquid crystal composition comprising at least one liquid crystalline compound as described in any one of the items [1] to [13]. The embodiment thereof is described in the following items [15] to [21].
[15] The liquid crystal composition as described in the item [14], further comprising at least one compound selected from the group consisting of compounds represented by Formulas (2), (3) and (4) as a second component: 
wherein R3 represents an alkyl group having 1 to 10 carbon atoms, in which any xe2x80x94CH2xe2x80x94 not adjacent to each other may be substituted with xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94 and any hydrogen may be substituted with fluorine; X1 represents fluorine, chlorine, xe2x80x94OCF3, xe2x80x94OCF2H, xe2x80x94CF xe2x80x94CF2H, xe2x80x94CFH2, xe2x80x94OCF2CF OCF2CFHCF3; L1 and L2 each independently represent hydrogen or fluorine; Z5 and Z6 each independently represent xe2x80x94(CH2)2xe2x80x94, xe2x80x94(CH2)4xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or a single bond; rings A and B each independently represent 1,4-cyclohexylene, 1,3-dioxane-2,5-diyl, or 1,4-phenylene in which hydrogen may be substituted with fluorine; and ring C represents 1,4-cyclohexylene, or 1,4-phenylene in which hydrogen may be substituted with fluorine.
[16] The liquid crystal composition as described in the item [14], further comprising at least one compound selected from the group consisting of compounds represented by Formulas (5) and (6) as a second component: 
wherein R4 and R5 each independently represent an alkyl group having 1 to 10 carbon atoms, in which any xe2x80x94CH2xe2x80x94 not adjacent to each other may be substituted with xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94 and any hydrogen may be substituted with fluorine; X2 represents xe2x80x94CN or xe2x80x94Cxe2x89xa1xe2x80x94Cxe2x80x94CN; ring D represents 1,4-cyclohexylene, 1,4-phenylene, 1,3-dioxane-2,5-diyl or pyrimidine-2,5-diyl; ring E represents 1,4-cyclohexylene, 1,4-phenylene in which hydrogen may be substituted with fluorine, or pyrimidine-2,5-diyl; ring F represents 1,4-cyclohexylene or 1,4-phenylene; Z7 represents xe2x80x94(CH2)2xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94 or a single bond; L3, L4 and L5 each independently represent hydrogen or fluorine; and b, c and d each independently represent 0 or 1.
[17] The liquid crystal composition as described in the item [14], further comprising at least one compound selected from the group consisting of compounds represented by Formulas (7), (8) and (9) as a second component: 
wherein R6 and R7 each independently represent an alkyl group having 1 to10 carbon atoms, in which any xe2x80x94CH2xe2x80x94 not adjacent to each other may be substituted with xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94 and any hydrogen may be substituted with fluorine; rings G and I each independently represent 1,4-cyclohexylene or 1,4-phenylene; L6 and L7 each independently represent hydrogen or fluorine, but L6 and L7 are not hydrogens at the same time; and Z8 and Z9 each independently represent xe2x80x94(CH2)2xe2x80x94, xe2x80x94COOxe2x80x94 or a single bond.
[18] The liquid crystal composition as described in the item [14], further comprising at least one compound selected from the group consisting of the compounds represented by Formulas (2), (3) and (4) as a second component and at least one compound selected from the group consisting of compounds represented by Formulas (10), (11) and (12) as a third component: 
wherein R8 and R9 each independently represent an alkyl group having 1 to 10 carbon atoms, in which any xe2x80x94CH2xe2x80x94 not adjacent to each other may be substituted with xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94 and any hydrogen may be substituted with fluorine; rings J, K and M each independently represent 1,4-cyclohexylene, pyrimidine-2,5-diyl, or 1,4-phenylene in which hydrogen may be substituted with fluorine; and Z10 and Z11 each independently represent xe2x80x94Cxe2x89xa1xe2x80x94Cxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94(CH2)2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or a single bond.
[19] The liquid crystal composition as described in the item [14], further comprising at least one compound selected from the group consisting of the compounds represented by Formulas (5) and (6) as a second component and at least one compound selected from the group consisting of the compounds represented by Formulas (10), (11) and (12) as a third component.
[20] The liquid crystal composition as described in the item [14], further comprising at least one compound selected from the group consisting of the compounds represented by Formulas (7), (8) and (9) as a second component and at least one compound selected from the group consisting of the compounds represented by Formulas (10), (11) and (12) as a third component.
[21] The liquid crystal composition as described in the item [14], further comprising at least one compound selected from the group consisting of the compounds represented by Formulas (2), (3) and (4) as a second component, at least one compound selected from the group consisting of the compounds represented by Formulas (5) and (6) as a third component and at least one compound selected from the group consisting of the compounds represented by Formulas (10), (11) and (12) as a fourth component.
The third aspect of the present invention relates to:
[22] A liquid crystal composition comprising at least one liquid crystal composition as described in any one of the items [14] to [21], and further, at least one optically active compound.
The fourth aspect of the present invention relates to:
[23] A liquid crystal display element comprising the liquid crystal composition as described in any one of the items [14] to [22].
The compound of the present invention represented by Formula (1) is characterized by having a moiety in which xe2x80x94Oxe2x80x94 in a difluoropropyleneoxy group is bonded directly to a 1,4-phenylene group which may be substituted with fluorine. The compound of Formula (1) in which ring A3 is a 1,4-cyclohexylene group has a high clearing point, and the compound in which ring A3 is a 1,4-phenylene group has a large absolute value of dielectric anisotropy. Further, the compound of Formula (1) in which both Y1 and Y2 are hydrogens shows a neutral or positive dielectric anisotropy. Especially, the compound in which at least one of Y3 and Y4 is fluorine shows a large positive dielectric anisotropy. For example, the compound of the present invention (compound No. 40) shown in the example described later has a dielectric anisotropy xcex94xcex5 of 13.7, which is a much larger value than that of a compound in which a bonding group corresponding to a difluoropropyleneoxy group is a single bond ((13-1): xcex94xcex5=9.7).
On the other hand, the compound of Formula (1) in which both Y1 and Y3 are fluorines and both Y2 and Y4 are hydrogens shows a large negative dielectric anisotropy. Further, the compound of Formula (1) shows almost the same refractive anisotropy, while having a high clearing point, as compared with the compound in which a bonding group corresponding to a difluoropropyleneoxy group in the present invention is a single bond. As seen from these matters, the compound of Formula (1) has suitable characteristics as a liquid crystalline compound constituting a liquid crystal composition for a VA mode or an MVA mode as well as a reflective liquid crystal display element described in the Background of the Invention.
Compounds having a difluoropropyleneoxy group as a bonding group are partly described in WO97/37959 and represented by a general Formula similar to that of the present invention, but any structural Formula or physical properties of a specific compound is not disclosed. The excellent characteristics of the compound of the present invention described above have newly been found by the present inventors.
The liquid crystalline compound represented by Formula (1) is classified into the group of the compounds represented by Formulas (1-1) to (1-6) by suitably selecting k, l, m and n: 
wherein R1, R2, rings A1 to A5, Z1 to Z4 and Y1 to Y4 have the same meanings as described above.
Among these compounds of more specific concept, a compound of two-ring system represented by Formula (1-1) has a relatively large absolute value of xcex94xcex5 and a relatively small xcex94n, and it has a low viscosity and a good compatibility at low temperature. When this compound is used as a component for a liquid crystal composition, a viscosity of the composition can be reduced while maintaining an absolute value of xcex94xcex5 thereof, and therefore, a liquid crystal composition for high-speed response can be provided.
Further, a compound of three-ring system represented by Formula (1-2) or (1-3) has a large absolute value of xcex94xcex5 and a relatively small xcex94n, and it shows a liquid crystal phase in a relatively wide temperature range. This compound as a component for a liquid crystal composition can elevate an absolute value of xcex94xcex5 of the composition without lowering a clearing point thereof, and therefore, it can provide a liquid crystal composition for driving a display element at a low voltage.
Compounds of four-ring system represented by Formulas (1-4) to (1-6) have a large absolute value of xcex94xcex5 and show a relatively small xcex94n. Further, a liquid crystal phase is shown at high temperature in the wide range. Accordingly, when this compound is used as a component for a liquid crystal composition, an absolute value of xcex94xcex5 can be increased, and a liquid crystal phase temperature range shown by the liquid crystal composition can be extended to a high temperature range.
Further, the compounds of these Formulas in which at least one of Y1, Y2, Y3 and Y4 is fluorine are characterized by having an excellent compatibility at low temperature.
In the compound represented by Formula (1), R1 and R2 each independently represent hydrogen, halogen, a cyano group, or an alkyl group having 1 to 20 carbon atoms, in which xe2x80x94CH2xe2x80x94 may be substituted with xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94 but xe2x80x94Oxe2x80x94 is not adjacent to another xe2x80x94Oxe2x80x94, and in which at least one hydrogen may be substituted with halogen; rings A1 to A5 each independently represent a 1,4-cyclohexylene group in which at least one xe2x80x94CH2xe2x80x94 not adjacent to each other may be substituted with xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94, a 1,4-cyclohexenylene group, or a 1,4-phenylene group in which at least one xe2x95x90CHxe2x80x94 may be substituted with =Nxe2x80x94 and hydrogen on the ring may be substituted with halogen; Z1 to Z4 each independently represent a single bond, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94CF2Oxe2x80x94 or xe2x80x94OCF2xe2x80x94; Y1, Y2, Y3 and Y4 each independently represent hydrogen or fluorine; and k, l, m and n each independently represent 0 or 1.
Specifically, R1 and R2 represent hydrogen, halogen, a cyano group, an alkyl group, an alkoxy group, an alkoxyalkyl group, an alkylthio group, an alkylthioalkyl group, an alkenyl group, an alkenyloxy group, an alkenylthio group, an alkynyl group, a fluoro-substituted alkyl group, a fluoro-substituted alkoxy group, a fluoro-substituted alkenyl group, a fluoro-substituted alkenyloxy group, a fluoro-substituted alkenylthio group and a fluoro-substituted alkynyl group.
More specifically, they represent fluorine, chlorine, bromine, methyl, ethyl, propyl, isopropyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, methoxy, ethoxy, propoxy, butoxy, pentoxy, heptyloxy, octyloxy, methoxymethyl, ethoxymethyl, propoxymethyl, propoxyethyl, methoxypropyl, ethoxypropyl, propoxypropyl, methylthio, ethylthio, propylthio, butylthio, pentylthio, hexylthio, heptylthio, octylthio, methylthiomethyl, ethylthiomethyl, propylthiomethyl, butylthiomethyl, methylthioethyl, ethylthioethyl, propylthioethyl, methylthiopropyl, ethylthiopropyl, propylthiopropyl, vinyl, 1-propenyl, 1-butenyl, 1-pentenyl, 3-butenyl, 3-pentenyl, ethynyl, 2-propynyl, 2-butynyl, 3-butynyl, 3-pentynyl, allyloxy, trifluoromethyl, fluoromethyl, 2-fluoroethyl, difluoromethyl, 2,2,2-trifluoroethyl, 1,1,2,2-tetrafluoroethyl, 2-fluoroethyl, 3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, fluoromethoxy, trifluoromethoxy, difluoromethoxy, pentafluoroethoxy, 1,1,2,2-tetrafluoroethoxy, heptafluoropropoxy, 1,1,2,3,3,3-hexafluoropropoxy, trifluoromethoxymethyl, 2-fluoroethenyl, 2,2-difluoroethenyl, 1,2,2-trifluoroethenyl, 3-fluoro-1-butenyl, 4-fluoro-1-butenyl, trifluoromethylthio, difluoromethylthio, 1,1,2,2-tetrafluoroethylthio, 2,2,2-trifluoroethylthio and the like.
Specifically, rings A1 to A5 preferably have ring structures represented by Formulas (r-1) to (r-24): 
The liquid crystal composition of the present invention may comprise only the first component comprising at least one liquid crystalline compound represented by Formula (1), and preferably, it may further comprises as a second component at least one compound (hereinafter referred to as second component A) selected from the group consisting of the compounds represented by Formulas (2), (3) and (4) described above and/or at least one compound (hereinafter referred to as second component B) selected from the group consisting of the compounds represented by Formulas (5) and (6). Further, the composition can comprise at least one compound selected from the group consisting of the compounds represented by Formulas (7), (8) and (9) as a third component for the purpose of controlling a threshold voltage, a liquid crystal temperature range, a refractive anisotropy, a dielectric anisotropy, a viscosity, etc. The respective components of the liquid crystal composition used for the present invention make little difference in physical properties, and therefore, they may be analogues comprising isotopes of the respective elements.
Among the compounds represented by Formulas (2), (3) and (4) as the second component A described above, preferable examples are the following compounds of Formulas (2-1) to (2-9), (3-1) to (3-97) and (4-1) to (4-33), respectively. 
In Formulas, R3 and X1 have the same meanings as described above.
These compounds represented by Formulas (2) to (4) show a positive dielectric anisotropy and are very excellent in heat stability and chemical stability, so that they are used primarily for liquid crystal compositions for TFT. When preparing a liquid crystal composition for TFT, an amount of the above compounds may preferably be in the range of 1 to 99% by weight, more preferably 10 to 97% by weight and further preferably 40 to 95% by weight based on the total amount of the liquid crystal composition. The compounds represented by Formulas (10) to (12) may further be added for the purpose of controlling the viscosity.
Among the compounds represented by Formulas (5) and (6) as the second component B described above, preferable examples are the following compounds of Formulas (5-1) to (5-58) and (6-1) to (6-3), respectively. 
In Formulas, R4, R5 and X2 have the same meanings as described above.
These compounds represented by Formulas (5) and (6) have a positive dielectric anisotropy, and a value thereof is very large, so that they are used mainly for liquid crystal compositions for STN and TN. These compounds are used as a component of the composition particularly for the purpose of reducing a threshold voltage. Further, they are used for the purposes of controlling the viscosity and the refractive anisotropy, expanding the liquid crystal phase temperature range, and also improving the steepness. When preparing a liquid crystal composition for STN or TN, an amount of the compounds of the Formulas (5) and (6) may preferably be in the range of 0.1 to 99.9% by weight, more preferably 10 to 97% by weight and further preferably 40 to 95% by weight. A third component described later can be mixed for the purpose of controlling the threshold voltage, the liquid crystal phase temperature range, the refractive anisotropy, the dielectric anisotropy, the viscosity, etc.
When preparing a liquid crystal composition for a vertical alignment mode (VA mode), etc. having a negative dielectric anisotropy, preferable is the composition comprising at least one compound (hereinafter referred to as second component C) selected from the group consisting of the compounds represented by Formulas (7) to (9). Preferable examples of the compounds represented by Formulas (7) to (9) among the second component C are the following compounds of Formulas (7-1) to (7-3), (8-1) to (8-5) and (9-1) to (9-3), respectively. 
In Formulas, R6 and R7 have the same meanings as described above.
The compounds represented by Formulas (7) to (9) have a negative dielectric anisotropy. The compound of Formula (7) is a compound of two-ring system and therefore used mainly for the purpose of controlling the threshold voltage, the viscosity or the refractive anisotropy. The compound of Formula (8) is used for the purpose of not only expanding the nematic range but also reducing the threshold voltage and increasing the refractive anisotropy.
The compounds represented by Formulas (7) to (9) are used mainly for a liquid crystal composition for a VA mode having a negative dielectric anisotropy. If an amount thereof is increased, the composition has a reduced threshold voltage but it may also have an increased viscosity. Therefore, an amount thereof is preferably reduced as long as a required value of the threshold voltage is satisfied. However, an absolute value of the dielectric anisotropy is 5 or less, and if an amount of the compounds represented by Formulas (7) to (9) are less than 40% by weight, the liquid crystal composition may not be driven. Accordingly, an amount of the compounds represented by Formulas (7) to (9) is preferably 40% by weight or more when preparing the composition for a VA mode. It is more preferably in the range of 50 to 95% by weight.
Further, the compounds represented by Formulas (7) to (9) may be added to the liquid crystal composition having a positive dielectric anisotropy for the purpose of controlling the elastic constant and a voltage transmission curve of the composition. In this case, an amount of the compounds represented by Formulas (7) to (9) is preferably 30% by weight or less.
Among the compounds represented by Formulas (10) to (12) as the third component for the liquid crystal composition of the present invention, preferable examples are the following compounds of Formulas (10-1) to (10-11), (11-1) to (11-12) and (12-1) to (12-6), respectively. 
In Formulas, R8 and R9 have the same meanings as described above.
The compounds represented by Formulas (10) to (12) have a small absolute value of a dielectric anisotropy and are almost neutral. The compound of Formula (10) is used mainly for the purpose of controlling the viscosity or the refractive anisotropy. The compounds of Formulas (11) and (12) are used for the purpose of expanding the nematic range by elevating the clearing point, or controlling the refractive anisotropy.
If an amount of the compounds represented by Formulas (10) to (12) is increased, the liquid crystal composition has an elevated threshold voltage and a reduced viscosity. Therefore, the compounds represented by the Formulas (10) to (12) are preferably used in a large amount in order to make low voltage driving possible as long as a required value of the threshold voltage of the liquid crystal composition is satisfied. When preparing the liquid crystal composition for TFT, an amount of the compounds represented by Formulas (10) to (12) is preferably 40% by weight or less, more preferably 35% by weight or less. When preparing the liquid crystal composition for STN or TN, an amount of the compounds represented by Formulas (10) to (12) is preferably 70% by weight or less, more preferably 60% by weight or less.
The liquid crystal composition of the present invention preferably comprises at least one of the liquid crystalline compounds represented by Formula (1) in the proportion of 0.1 to 99% by weight in order to make low voltage driving possible.
The above liquid crystal composition is usually prepared by a publicly known method, for example, a method of dissolving various components at high temperature. Further, a chiral dopant is added if necessary, whereby the liquid crystal composition can be improved for each purpose and optimized. Any chiral dopant may be used as long as it can induce a helical structure of liquid crystals to adjust a required twist angle and prevent reverse twist. For example, the following optically active compounds can be given as the chiral dopant. 
In the liquid crystal composition of the present invention, these optically active compounds are usually added to adjust a pitch of twist. The pitch of twist is preferably adjusted in the range of 40 to 200 xcexcm in the case of the liquid crystal compositions for TFT and TN. In the case of the liquid crystal composition for STN, it is preferably adjusted in the range of 6 to 20 xcexcm. Further, in the case of the liquid crystal composition for a bistable TN mode, it is preferably adjusted in the range of 1.5 to 4 xcexcm. Two or more optically active compounds may be added for the purpose of controlling a temperature dependency of the pitch.
The liquid crystal composition of the present invention can also be used as a liquid crystal composition for a G-H mode by adding a dichroic dye such as merocyanine, styryl, azo, azomethine, azoxy, quinophthalone, anthraquinone and tetrazine. The composition according to the present invention can also be used as a liquid crystal composition for NCAP prepared by the micro-encapsulation of nematic liquid crystals, for a polymer dispersed liquid crystal display element (PDLCD) such as a polymer network liquid crystal display element (PNLCD) in which a three-dimensional polymer is formed in liquid crystals, and for an electrically controlled birefringence mode (ECB) or DS mode liquid crystal displays.
The compounds represented by Formula (1) according to the present invention can be produced by suitably selecting and combining methods described in publications or publicly known literatures on organic synthesis such as Jikken Kagaku Koza (Experimental Chemistry Course) 4th Edition (Maruzen), Organic Synthesis (John Wiley and Sons, Inc.) or Organic Reactions (John Wiley and Sons, Inc.).
The compound represented by Formula (1) can be prepared by the following method, for example. First, a propionic acid ester derivative (16) obtained by a method described in JP-A 59-76027, JP-A 60-197637 or JP-A 60-204743 is converted to a thioester derivative (17) with a Lawesson""s reagent (Fieser 13, 38) according to a method disclosed in JP-A 10-204016. Further, the derivative (17) it) is reacted with HF-pyridine in the presence of an oxidizing agent such as N-bromosuccinimide (hereinafter abbreviated as NBS) according to a method disclosed in JP-A 5-255165. 
In Formulas, R1, R2, rings A1 to A5, Z1 to Z4, Y1 to Y4, k, l, m and n have the same meanings as described above.
The compound represented by Formula (1) can suitably be prepared by the following method as well. According to a method described in JP-A 10-17544, a Grignard reagent is prepared from a halobenzene derivative (18) and then reacted with carbon disulfide to obtain a dithiocarboxylic acid derivative (19). The derivative (19) is then reacted with sodium hydride in the presence of a phenol derivative (20) and further oxidized with iodine to obtain the thioester derivative (17). The derivative (17) thus obtained is reacted with HF-pyridine in the presence of an oxidizing agent such as NBS to give the desired compound (1). 
In Formulas, R1, R2, rings A1 to A5, Z1 to Z4, Y1 to Y4, k, l, m and n have the same meanings as described above, hydrogen on a benzene ring may be substituted with fluorine, and X represents chlorine or bromine.
The phenol derivative (20) used above can be prepared according to a method of R. L. Kidwell, et al. (Org. Synth., V, 918 (1973)), for example, by reacting trialkyl borate with a Grignard reagent prepared from a benzene derivative (21) to give a boric acid ester derivative and then oxidizing this derivative with a peroxide such as hydrogen peroxide and peracetic acid. 
In Formulas, R2, rings A4 and A5, Z3, Z4, Y1 to Y4, m and n have the same meanings as described above, R10 represents an alkyl group, and X1 represents a chlorine atom or a bromine atom.
The compound represented by Formula (1) can preferably be prepared by the following method as well. Specifically, carboxylic acid derivative (22) is reacted with propanedithiol and a strong acid such as trifluoroacetic acid in a suitable solvent such as toluene to obtain a dithianium salt (23). The reaction is carried out at room temperature to the boiling point of the solvent, and preferably at 100xc2x0 C. or higher with removing produced water to isolate the salt in a stable form. The salt (23) can be tetrafluoroborate or perchlorate in addition to dithianium trifluoromethanesulfonate. Subsequently, the salt (23) is reacted with a base such as triethylamine in a suitable solvent such as methylene chloride in the presence of a phenol derivative (20) to obtain dithioortho ester derivative (24). The derivative (24) is then oxidatively fluorinated to obtain the compound (1) by reacting with a fluorinating agent such as triethylamine-3HF, and further, with an oxidizing agent such as NBS and bromine. The reaction is carried out at lower temperature, preferably at xe2x88x92100xc2x0 C. to xe2x88x9250xc2x0 C. for practical reasons. 
In Formulas, R1, R2, rings A1 to A5, Z1 to Z4, Y1 to Y4, k, l, m and n have the same meanings as described above, and hydrogen on a benzene ring may be substituted with fluorine.
The present invention shall further be explained below in details with reference to examples.
In the respective examples, Cr represents crystal, N represents a nematic phase, and Iso represents an isotropic liquid phase.